Method for reducing freeze-thaw voids in uncured adhesives

ABSTRACT

A method for reducing the level of freeze-thaw voids in an uncured adhesive subjected to freezing and thawing comprises storing the adhesive in a container in which the walls of the container are a thermoplastic material and have been thinned and roughened.

FIELD OF THE INVENTION

This invention relates to a method for reducing the number of freezethaw voids in uncured adhesives and to containers for storing uncuredadhesives at temperatures near or below the freezing point of theadhesive.

BACKGROUND OF THE INVENTION

When an uncured adhesive is stored at a temperature near or below itsfreezing point, and is then brought into ambient temperatures forthawing, air voids can form between the wall of the container and theadhesive. This occurs as the container warms faster than the adhesive,expands and pulls away from the adhesive, introducing a space betweenthe adhesive and the container wall. This phenomenon is often referredto as delamination between the adhesive and container. Upon thawing, asthe adhesive re-wets the walls of the container, air located between thecontainer and adhesive may become entrapped. The amount of air entrappedis related to the number of freeze/thaw cycles to which the adhesive issubjected. Within the microelectronics industry, uncured adhesive iscommonly shipped in syringes under temperature conditions below thefreezing point of the adhesive. If air voids form within the uncuredadhesive, the voids can cause incomplete dispense patterns and tailingwhen the adhesive is dispensed. This type of void is commonly referredto as a freeze-thaw void (FTV). Another problem that can occur duringthe freeze/thaw cycle is cracking within the bulk of the adhesive. Whenthis happens, air can be introduced into the cracks. Upon thaw, the airaccumulates into micro bubbles that cause the same issues as FTVs,however in this case the air is scattered throughout the bulk adhesive.

This invention comprises a solution to the above problem by providing acontainer that mechanically or chemically increases the bonding strengthof the frozen, uncured adhesive to the walls of the container, and/orthat is sufficiently compliant to allow the adhesive to remain incontact with the container walls during freeze-thaw cycles.

SUMMARY OF THE INVENTION

This invention is a method for reducing the level of freeze-thaw voidsin an uncured adhesive subjected to freezing and thawing comprisingstoring the adhesive in a container in which the walls of the containerare a thermoplastic material and (i) have a thickness of 0.0254 mm to0.762 mm (1 to 30 mils) or (ii) have a thickness of 0.0254 to 1.524 mm(1 to 60 mils) and are roughened to have a mean roughness value ofgreater than 0.3 μm. In another embodiment, this invention is acontainer suitable for containing an uncured adhesive at a temperaturenear or below the freezing point of the adhesive in which the walls ofthe container are a thermoplastic material and (i) have a thickness of0.0254 mm to 0.762 (1 to 30 mils) or (ii) have a thickness of 0.0254 to1.524 mm (1 to 60 mils) and are roughened to have a mean roughness valueof greater than 0.3 μm. In a preferred embodiment, the container is asyringe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the effects of roughening on the FTV performance ofadhesive A while using a syringe with a wall thickness of 0.762 mm.

FIG. 2 shows the effects of wall thickness on the FTV performance ofadhesive A while maintaining a roughness (Ra) of 2.9 microns.

FIG. 3 shows the effects of roughening on the FTV performance ofadhesive B while using a syringe with a wall thickness of 1.524 mm.

FIG. 4 shows the effects of wall thickness on the FTV performance ofadhesive B while maintaining a roughness (Ra) of 2.9 microns.

DETAILED DESCRIPTION OF THE INVENTION

The thinness of the walls of the container makes the container morecompliant or flexible. As thawing of the adhesive occurs, the walls ofthe container expand faster than the adhesive. The added flexibilityallows the container to move with the adhesive and inhibit the creationof space between the adhesive and the walls. Similarly, a very compliantsyringe material will be more likely to conform to the adhesive as itshrinks and expands than a less compliant syringe material, thusreducing the likelihood of delamination and the formation of FTVs. Theroughness of the inside walls of the container increases mechanicalbonding of the adhesive to the walls. The mean roughness value (R_(a))is the surface texture of the walls of the container as measured with asurface profilometer. It is the arithmetic average of the absolutevalues of the roughness profile ordinates; that is, the average heightof the contours that creates the roughness. In order for adhesion to beincreased sufficiently to reduce the formation of air voids, the R_(a)value should be greater than 0.3 μm for containers prepared ofthermoplastic materials.

Roughening or adding contours to the inside container walls can beaccomplished by any method effective for obtaining the necessary R_(a)value, for example, by adding contours to the interior walls of thecontainer during fabrication, mechanical abrasion, plasma etching,chemical etching or corona discharge, either during or afterconstruction of the container. The syringe mold can be made rougher,resulting in a rougher syringe surface, by reducing the level of polishused on the mold surface or by adding contours or bumps to the syringewall surfaces. Mechanical abrasion can be accomplished, for example, byrubbing the inside of the syringe with sand paper, sandblasting it, orscraping or scoring it with a tool, as well as any other method ofabrasion that would result in an increased surface roughness.

Plasma etching could be performed on the container using a variety ofgasses, including SF₆, O₂, Ar, CF₄, CHF₃, and O₃. The container could betreated via corona discharge using conditions known in the art, at anintensity and duration dictated by the desired level of roughening.

Alternatively, the roughening can be accomplished by chemically etchingthe surface using a chromic acid solution, in which the container wouldbe immersed in the solution for 60-90 minutes at ambient temperature or1-2 minutes at 65-70° C. The chromic acid solution should be composed ofsodium dichromate (15 parts/weight), distilled water (24 parts/weight),and concentrated sulfuric acid (300 parts/weight).

In another method, adhesion can be enhanced chemically by coating theinside of the container with a primer for plastic material. Suitableprimers for polypropylene-based containers include chlorinatedpolypropylene primers such as SUPERCHLON S-3199 available from NipponUnipac Holding Group, non-chlorinated primers such as Eastman AP 440-1available from Eastman Chemical Company, and acrylic enamels such asAbrex 44-series acrylic enamels available from Abrex Paint and ChemicalLtd. Suitable primers for polyethylene-based containers include primerssuch as Eastman chlorinated polyolefin CP 153 available from EastmanChemical Company and acrylic enamels such as Abrex 44-series acrylicenamels available from Abrex Paint and Chemical Ltd. Such primers may beapplied by spraying or brushing it on, by spin coating, or by dipping.

Preferred thermoplastic materials are injection moldable and have aflexural modulus of less than or equal to 1240 MPa (180,000 psi).Suitable materials are selected from the group consisting ofpolypropylene, polyethylene, ethylene-ethyl acrylate copolymer,ethylene-vinyl acetate copolymer, high density polyethylene, low densitypolyethylene, ethylene-octene copyolymer, ethylene-hexene copolymer,ethylene-butene copolymer, polypropylene homopolymer, polypropylenecopolymer, and polypropylene random copolymer. These materials arecommercially available and can be obtained, for example, from The DowChemical Company, E.I. du Pont de Nemours and Company, ExxonMobil, orUnion Carbide Corporation.

Embodiments of the invention can vary as the dispensing needs dictate.In one embodiment the preferred thermoplastic container is athermoplastic syringe. In another embodiment the preferred thermoplasticcontainer is a compliant syringe used within a more rigid sleeve.

Testing was conducted by filling 10 cc sized polypropylene syringes withadhesive. Two exemplary adhesive chemistry types were tested. Adhesive Awas ABLEFILL UF8822 underfill encapsulant, which is based onmoisture-resistant cyanate ester chemistry and has a freezing point of−17° C. Adhesive B was ABLEBOND 84-3MVBTI adhesive, which is based onepoxy chemistry and has a freezing point of −38° C. Both adhesives arecommercially available from Ablestik Laboratories. The syringes wereplaced in a variety of freezers to achieve a range of storagetemperatures below the freezing point of the adhesive. Syringetemperature was measured using a thermocouple attached to the side ofthe syringe in the middle of the length of the barrel. The material wasallowed to remain frozen for a minimum of 2 hours and was then removedfrom the freezer and allowed to set at room temperature (20-25° C.)until the temperature of the syringe was at the freezing point of theadhesive. The syringe was then visually examined for cracks, freeze/thawvoids, or delamination. After the adhesive reached room temperature thesyringe was visually examined for freeze-thaw voids. Any level ofcracking, delamination, or freeze-thaw voids was considered a failurefor that specimen. Ten specimens were tested for each data point and thepercentage of specimen failure was recorded.

Results are presented in FIGS. 1-4. FIG. 1 shows the effects ofroughening on the FTV performance of adhesive A while using a syringewith a wall thickness of 0.762 mm. In the example below, increasing theroughness (R_(a)) from 0.1 microns to 2.9 microns decreased the numberof failed syringes from 100% to less than 10% for the temperature rangebetween −67° C. to −36° C.

FIG. 2 shows the effects of wall thickness on the FTV performance ofadhesive A while maintaining a roughness (Ra) of 2.9 microns. In theexample below, as the wall thickness was decreased from 1.524 mm to0.762 mm, the percentage of failed syringes dropped from approximately90% to less than 10% for the temperature range between −67° C. to −36°C.

FIG. 3 shows the effects of roughening on the FTV performance ofadhesive B while using a syringe with a wall thickness of 1.524 mm. Asthe roughness was increased, the percentage of failed syringesdecreases. In the example below, increasing the roughness (Ra) from 0.1microns to 2.9 microns decreased the number of failed syringes fromapproximately 65% to less than 20% for the temperature range between−65° C. to −45C.

FIG. 4 shows the effects of wall thickness on the FTV performance ofadhesive B while maintaining a roughness (Ra) of 2.9 microns. In theexample below, as the wall thickness was decreased from 1.524 mm to0.762 mm, the percentage of failed syringes dropped from approximately16% to less than 2% for the temperature range between −65° C. to −45° C.

1. A method for reducing the level of freeze-thaw voids in an uncuredadhesive subjected to freezing and thawing comprising storing theadhesive in a container in which the walls of the container are athermoplastic material and (i) have a thickness of 0.0254 mm to 0.762 mmor (ii) have a thickness of 0.0254 to 1.524 mm and are roughened to havea mean roughness value of greater than 0.3 μm.
 2. The method accordingto claim 1 in which the thermoplastic material is injection moldable andhas a flexural modulus of less than or equal to 1240 MPa.
 3. The methodaccording to claim 2 in which the thermoplastic material is selectedfrom the group consisting of polyethylene, ethylene-ethyl acrylatecopolymer, ethylene-vinyl acetate copolymer, high density polyethylene,low density polyethylene, ethylene-octene copyolymer, ethylene-hexenecopolymer, ethylene-butene copolymer, polypropylene homopolymer,polypropylene copolymer, and polypropylene random copolymer.
 4. Themethod according to claim 1 in which the container is a syringe or asyringe within a rigid sleeve.
 5. The method according to claim 1 inwhich the container has walls having a thickness of 0.0254 mm to 0.762mm.
 6. The method according to claim 1 in which the container has wallshaving a thickness of 0.0254 to 1.524 mm and are roughened to have amean roughness value of greater than 0.3 μm.
 7. The method according toclaim 1 in which the walls of the container are roughened by: addingcontours to the interior walls of the container during fabrication,mechanical abrasion, plasma etching, chemical etching, corona discharge.8. A container in which the walls of the container are a thermoplasticmaterial and (i) have a thickness of 0.0254 mm to 0.762 mm or (ii) havea thickness of 0.0254 to 1.524 mm and are roughened to have a meanroughness value of greater than 0.3 μm.
 9. The container according toclaim 8 in which the container is a syringe or a syringe within a rigidsleeve.